The invention relates to a process for the simultaneous production of fructose and of gluconic acid from glucose-fructose mixtures and/or from invert sugar.
Although this problem has existed for a long time, it has not as yet been solved in a technically flawless way.
U.S. Pat. No. 2,567,060 and DOS [German Unexamined Laid-Open Application] 2,130,994 (published on Dec. 30, 1971) disclose subjecting a mixture of glucose (dextrose) and fructose to an electrochemical oxidation wherein glucose is oxidized to gluconic acid and separated as sparingly soluble calcium gluconate; fructose is isolated from the mother liquor. The fructose yield is relatively low, since a substantial proportion thereof is destroyed under the conditions of the electrochemical oxidation in a weakly alkaline medium.
In a similar fashion, attempts have been made to convert invert sugar into calcium gluconate and fructose by selective oxidation with bromine in the presence of calcium hydroxide. This process is expensive and cumbersome; it requires the regeneration of the bromine and, additionally, an essential portion of the fructose is also destroyed in this process.
It is known that fructose is very delicate. In strongly salt-containing solutions, fructose is already unstable, especially under heat; alkalis effect a very rapid destruction of fructose.
To preclude the destruction of fructose by the effect of the oxidizing agent, it has been suggested to subject glucose to a biochemical oxidation in a neutral to weakly acidic medium by the action of spores of Aspergillus niger [Murtaugh et al., U.S. Pat. No. 2,949,389; Sato et al., Chem. Abstr. 68:113323e (1968), and Kulhanek et al. Listy Cukrov 1972:88(2), 31-5 (Chem. Abstr. 77:21923h [1972])] or of Aerobacter aerogenes [Dalby et al., J. Microbiol. 1:733-42 (1955) (Chem. Abstr. 50:3517g [1956])].
However, this method is not optimally suited for technical production purposes. Too many by-products are encountered. The available microorganisms are furthermore inadequate in their efficiency. Also, a high dilution must be employed which considerably impedes the isolation of the readily water-soluble products.
Rao et al. (J. Electrochem. Soc. 1969:116[3], 334-7) describe the electrochemical oxidation of glucose to gluconic acid in a neutral medium. The thus-produced acid is, however, absorbed on the platinum-plated platinum electrode and prevents the continuance of electrolysis. This experiment showed that the electrochemical oxidation takes place satisfactorily only in an alkaline medium; according to experience, under all tested conditions, such a large proportion of the fructose is destroyed that this process cannot be considered from an economical viewpoint.